The present invention relates to the diagnostic medical imaging and spectroscopy arts. It finds particular application in conjunction with magnetic resonance imaging (MRI) scanners, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
Commonly, in MRI, a substantially uniform temporally constant main magnetic field, B0, is set up in an examination region in which a subject being imaged or examined is placed. Nuclei in the subject have a spin which in the presence of the main magnetic field produce a net magnetization. The nuclei of the spin system precess in the magnetic field at the Larmor frequency, i.e., the resonant frequency. Radio frequency (RF) magnetic fields at and/or near the resonant frequency are used to manipulate the net magnetization of the spin system. Among other things, RF magnetic fields at the resonant frequency are used to, at least partially, tip the net magnetization from alignment with the main magnetic field into a plane transverse thereto. This is known as excitation, and the excited spins produce a magnetic field, at the resonant frequency, that is in turn observed by a receiver system. Shaped RF pulses applied in conjunction with gradient magnetic fields are used to manipulate magnetization in selected regions of the subject and produce a magnetic resonance (MR) signal. The resultant MR signal may be further manipulated through additional RF and/or gradient field manipulations to produce a series of echos (i.e., an echo train) as the signal decays. The various echos making up the MRI signal are typically encoded via magnetic gradients set up in the main magnetic field. The raw data from the MRI scanner is collected into a matrix commonly known as k-space. Typically, each echo is sampled a plurality of times to generate a data line or row of data points in k-space. The echo or data line""s position in k-space (i.e., its relative k-space row) is typically determined by its gradient encoding. Ultimately, in an imaging experiment, by employing Inverse Fourier or other known transformations, an image representation of the subject is reconstructed from the k-space (or reciprocal space) data.
Often, MR reception systems use a significant signal gain factor (i.e., amplification and/or attenuation) in order to properly fill the range of an employed analog to digital converter (ADC). Previously developed RF reception systems, typically use a single gain factor selected based on a calibration signal. This single gain factor is used for the complete echo train even though an amount of signal received from the echo train can have a large dynamic range. That is to say that signals from an echo train are commonly subjected to a single gain factor (i.e., amplification or attenuation) to achieve a desired result such as improved signal to noise ratio (SNR), avoidance of signal overflow in the ADC, or the like. Yet, a single gain factor is not universally optimal, for all the echos of an echo train, or even for some single echos.
Consider, for example, one known type of MRI experiment, multiple contrast acquisition and imaging. Different tissues of the body have different pairs of relaxation properties that are characterized by a pair of time constants: T1 which is the spin-lattice relaxation time, and T2 which is the spin-spin relaxation time. Therefore, different images and visualization of different anatomical structures are obtained depending upon the time constant most heavily relied upon. In this regard, a T1 weighted image is one in which the intensity contrast between any two tissues in an image is due mainly to the T1 relaxation properties of the tissue, and a T2 weighted image is one in which the intensity contrast between any two tissues in an image is due mainly to the T2 relaxation properties of the tissue. In still another type of contrast image, where the repetition time (TR) for the pulse sequence is long relative to the T1 relaxation properties of all the tissues under consideration, the achieved image is said to be proton density (PD)-weighted in that it reflects an overall proton density distribution.
In an exemplary dual-contrast experiment, two sets of echos out of the same echo train are used to reconstruct two images having different tissue contrast weighting. Typically, a set of later-acquired, smaller-signal echos are used in reconstructing a T2-weighted image, while the set of earlier-acquired, larger-signal echos are used in reconstructing a PD-weighted image. Accordingly, a predetermined gain factor set small enough to accommodate the larger-signal echos without clipping is too small to achieve an optimal SNR for the smaller-signal echos. Conversely, a predetermined gain factor set high enough to optimize the SNR of the smaller-signal echos is too high to accommodate the larger-signal echos without clipping.
There are other kinds of MR sequences, such as single echo volume acquisitions, where the received signal is large compared to the dynamic range of the ADC used to digitize the signal. Usually the received signal has several bits of noise so the real signal is not subjected to digitization noise in the lowest bits. There are many small signals in the acquisition of an image and only a few large signals near the center of k-space. If two or more ADC channels are used, the large signal data can be acquired with one ADC, and gain corrected in the data stream, while the many smaller signals can be received with appropriate gain in another channel.
One type of approach used to address SNR concerns in such circumstances is known as the variable bandwidth method. See, e.g., Greenman, et al., xe2x80x9cBilateral Imaging Using Separate Interleaved 3D Volumes and Dynamically Switched Multiple Receiver Coil Arrays,xe2x80x9d MRM, (1998), Vol. 39, pp. 108-115, incorporated herein by reference. However, variable bandwidth methods have certain inherent limitations and/or drawbacks. For example, depending on how it is implemented, one may incur undersampling in k-space, and generally, the noise behavior become more difficult to interpret. Moreover, the method employs dynamic timing changes that, technically, are not trivial or easy to implement.
The present invention contemplates a new and improved gain selection technique which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of magnetic resonance imaging includes supporting a subject in an examination region of an MRI scanner. An MRI pulse sequence is applied to produce a detectable magnetic resonance signal in a selected region of the subject. The magnetic resonance signal includes a plurality of echos which are received. The plurality of received echos are subjected to a controllable gain factor such that at least two echos are subjected to different gain factors.
In accordance with another aspect of the present invention, a method of magnetic resonance imaging is provided. It includes supporting a subject in an examination region of an MRI scanner, and applying an MRI pulse sequence to produce a detectable magnetic resonance signal in a selected region of the subject. The magnetic resonance signal comprises one or more echos. The magnetic resonance signal is received and selectively subjected to a variable gain factor. The gain factor affected magnetic resonance signal is sampled into k-space as k-space data, and, thereafter, the k-space data is reconstructed into an image representation of the subject.
In accordance with another aspect of the present invention, an MRI scanner includes a main magnet that generates a substantially uniform temporally constant main magnetic field through an examination region wherein an object being imaged is positioned. The scanner also includes: a gradient magnetic field generator that produces magnetic gradients in the main magnetic field across the examination region, and an RF magnetic field generator which includes an RF transmitter that drives an RF coil which is proximate to the examination region. A sequence control manipulates the gradient magnetic field generator and the RF magnetic field generator to produce an MRI pulse sequence. The MRI pulse sequence produces a detectable magnetic resonance signal, including one or more echos, in the object. A reception system includes a receiver that receives and demodulates the echos, and a gain control that selectively subjects the echos to a variable gain factor. A reconstruction processor reconstructs images of the object from data collected via the reception system, and an output device produces a human viewable rendering of the images.
One advantage of the present invention is separately optimized gain factors individually selected and/or set for each echo or group of echos in various MRI experiments.
Another advantage of the present invention is a high SNR for all the echos or groups of echos collected in an MRI experiment, particularly for echos late in the echo train.
Yet another advantage of the present invention is a dynamically varying gain factor use in connection with a multi-contrast MRI experiment does not affect the noise behavior nor does it introduce undersampling or time limitation problems.
Another advantage of the present invention is that a selectable gain employed during echo acquisition in conjunction with multi-echo multi-contrast experiments improves the SNR of the later echos without changing the sampling linearity or receive bandwidth of the early echos.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.